There are many situations in which it is desirable to detect changes in broad band energy sources. For example, it is sometimes desirable to detect changes in the sound output of an operating machine that may be indicative of a problem, or of a condition being monitored. It is also desirable to detect, localize and classify long range contacts or sources of sound in passive surveillance of the sub-surface ocean. Currently, such information is discerned using two or more physically spaced, non-directional or directional receivers.
In the non-directional receiver case: one of the received signals is delayed by a variable amount, then correlated with the signal from the other receiver. The presence and direction of a source are then apparent from a peak in the correlation coefficient at the value of delay which corresponds to the difference in propagation time of the sound from the source to the two receivers. For example, when the difference is zero, then the source must lie in a line which is perpendicular to the line joining the two receivers. If the difference is non-zero, then the source must be on a hyperbola, the exact location of which depends on the delay difference, the distance between the receivers, and the speed of propagation. The disadvantage of this method is that it uses non-directional receivers, which are inherently incapable of discriminating against noise arriving from directions other than the source direction, and which therefore suffer a degraded ability to detect weak sources.
This disadvantage may be partially overcome by using directional receivers, and scanning them both in azimuth. Consider the distance apart of the two receivers and the distance to the source; if the receiver separation is large compared to the range of the source, then the azimuths from the source to the two receivers have no predictable relationship, and the signals from all possible azimuth pairs must be correlated to find a source at an arbitrary position. The disadvantage of this approach is the cost of the large number of correlations that are required.
Alternatively, the directional receivers may be spaced sufficiently closely that the azimuths from the source to the receivers are essentially equal. Then the number of required correlations is greatly reduced. However, the proximity of the two receivers brings with it a greater probability that the received noises--as well as the source signals --are correlated, leading again to degraded ability to detect weak sources.